The invention presented herein relates to arrangements for stabilization of the output of an injection laser (laser diode) when modulated with a wideband analog signal.
An acceptable laser output stabilization arrangement must correct for long and short term changes in laser diode operating characteristics if the laser diode is to be usable for modulation by a wideband analog signal. Over the long term, the slope of the curve of laser diode power output versus input current tends to decrease with the age of the laser diode. In addition, the lasing threshold current tends to increase with age.
Short term variations in the laser diode operating characteristics can occur in response to junction temperature changes. Junction temperature changes due to self-heating or ambient temperature fluctuations can cause continuous changes in the power output of the laser diode for fixed input current. These continuous changes are equivalent to a shift to the right of the power-current curve with increasing junction temperature.
In addition to the continuous changes, some single-mode laser diodes exhibit abrupt power-output fluctuations of a few percent of maximum power due to "hopping" between longitudinal modes of the laser cavity. In a given external optical setup, these mode hops occur at particular junction temperatures, with the junction temperature being dependent primarily on the recent history of junction self-heating.
Various feedback arrangements for stabilizing the laser diode output are disclosed in the prior art which involve monitoring the output of the laser diode by directing a portion of the light emitted from the front or back facets of the laser diode to a pin diode. Some laser diodes are provided with an internal diode for such use. In the case of laser diodes driven from digital data sources, the feedback arrangements are provided to maintain average power levels. Such arrangements, of course, only provide correction for long term changes in laser diode operating characteristics and do not provide immediate or continuous correction for abrupt power-output fluctuations due to "hopping" between longitudinal modes of the laser cavity or power level changes due to short term temperature fluctuations.
A paper entitled "Modulated Light Source For Recording With GaAlAs-Lasers" by M. Lutz, B. Reimer, and H. P. Vollmer which was presented at the "First International Congress on Advances in Non-Impact Printing Technologies" held at Venice, Italy in June 1981, discloses an electronic circuit employing the pin diode that is built into the laser diode assembly. A feedback arrangement is described wherein the pin diode monitors the light output from the back mirror of a laser diode to stabilize the light output of the laser diode. This paper recognizes the need for rapid stabilization of the laser output for printer applications. A control circuit is disclosed which keeps the laser diode operating all the time so a feedback signal is always present and is indicated as providing a feedback loop having a time constant that is much less than 300 nanoseconds. The rise and fall times for this circuit are given as 150 and 300 nanoseconds, respectively.
While the prior art appears to teach how one might obtain regulation of the output of a laser diode making it suitable for non-impact printers, such teachings fall short with respect to how satisfactory control of the laser diode output can be obtained for applications using analog input signals wherein feedback corrections in less than 40 nanoseconds are desired. The prior art also fails to teach a solution to the problem presented due to mode "hopping" between longitudinal modes of the laser cavity.